Under normal conditions, gas flow measurement makes use not only of knowledge concerning the temperature T and the pressure P, but also of two other magnitudes, namely the volume flow rate as measured at the temperature T and the pressure P, and the compressibility factor Z or the density .rho., which are thermodynamic magnitudes depending on the pressure, the temperature, and the composition of the gas.
Although volume flow rate is easily measured with known uncertainty that is linked directly to the means implemented, be they mechanical means such as spinner meters or piston meters, for example, or be they static means such as diaphragm flow meters or ultrasonic flow meters, in particular, the compressibility factor nevertheless remains a magnitude that is difficult to define and to measure.
Calculation of the compressibility factor requires state equations to be used that are not valid in high pressure domains (greater than 120 bar) and low temperature domains (less than -8.degree. C.), and it is generally measured by means of density sensors that give results that are expressed as discrete values (chromatographic analysis requires a period of 15 minutes to 20 minutes per test).
With a fuel gas, where the composition of the gas varies as do the values of pressure and temperature required in calculating the compressibility factor Z, non-negligible volume errors arise and can give rise to disputes or to disagreements between vendors and purchasers, particularly when it is necessary to reduce the measured volumes to normal conditions of pressure and temperature which may be defined by a local or a national authority or which may merely be defined by an industrial customer, for example.
An object of the present invention is to implement a method of measuring the characteristics of a such as pressure, temperature, and compressibility factor so as to enable the flow rate of the gas to be transmitted to a remote point without requiring human intervention, with only maintenance and inspection requiring limited use of an operator.